Prototype quillwort (Isoetes prototypus) COSEWIC assessment and status report: chapter 3

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Species Information

Name and classification

Morphological description

(For more technical details see Britton and Goltz 1991.)

Isoetes prototypus (Figure 1) is a diploid species (2n = 22). It is perennial, and has a bilaterally symmetrical two-lobed (rarely three-lobed) corm (Figure 2) that bears 10-25 (-75) leaves, ranging from 4-12 (-15) cm long (Figure 3). The leaves are very straight, rigid, quill-like and brittle, abruptly tapering from a swollen base and then gradually tapering to a sharp tip (Figure 4). The leaf colour is mainly dark green except for the reddish brown or chestnut-coloured base. There is still some debate as to whether or not this species has a velum. Spores are borne in the swollen bases of the leaves. The female spores (megaspores) are white, 425-575 (mean = 500) µm in diameter, and levigate (fairly smooth) with low meandiform markings sometimes reduced to mere molded mounds, and very pronounced triradial (three-spoked) and equatorial ridges (Figure 5). The ornamentation continues to the equatorial ridge (the “girdle”). The microspores are pale brown, lenticular (monolete), 23-32 (mean = 28) µm in length and covered with a complex network of spinulose fibres (Figure 6). Spores mature in summer (Britton and Goltz 1991).

When viewed from above, the largest plants of I. prototypus have crowns measuring 8.5 x 20 cm with a resulting oval shape, being somewhat laterally compressed.

Although identification of I. prototypus is most reliably based on spore morphology and chromosome numbers at this time, helpful macroscopic characters that resulted in an accurate identification of most plants of this species in the field were as follows:

• The “quills” are straight, stiff and very brittle, often breaking when pressure (e.g., from an outstretched hand) was applied by pushing downward on the tips of the leaves. Other taxa with straight quills, I. lacustris and plants of I. x harveyi with the I. x heterospora morphology, tended to flex considerably (almost like an archer’s bow), rather than break.
• There is very little white colour near the bases of the leaves of most plants of I. prototypus, the bases of the leaves usually having a chestnut colour, and the leaves otherwise mostly having a dark green colour. Straight-quilled plants of I. lacustris and plants of I. x harveyi with I. x heterospora morphology often tended to have a broad white-coloured zone near the bases of the leaves.

Based on spore morphology, I. prototypus is most likely to be confused with I. hieroglyphica (now considered a morphological variant of I. lacustris) and I. acadiensis. However, the megaspores and microspores of I. prototypus are smaller than those of I. hieroglyphica or I. acadiensis. The megaspore ornamentation of I. prototypus is of lower relief and less pronounced than in either I. acadiensis or I. hieroglyphica. Unlike I. prototypus, the megaspores of I. acadiensis lack an especially smooth area distal to the equatorial ridge.

Since chromosome counts were not done on any of the 2003 or 2004 collections, plants lacking megaspores were identified on the basis of their microspore size (which usually provides an accurate indication of ploidy level) and macroscopic characteristics.

Genetic description

Isoetes prototypus is a diploid species (2n = 22; Britton and Goltz 1991). Only one other diploid Isoetes species, I. echinospora, is known to occur in the Maritime provinces. Hybrids involving I. prototypus, if they occur, would have a chromosome number intermediate between their two parents. Research on the DNA of I. prototypus and other Isoetes species is presently ongoing.